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Effect of periodic surface structures (PSS) formation on a laser—irradiated target surface is still of essential interest since these structures are responsible for redistribution of laser energy flux on the target and thus affect the coupling efficiency. Moreover, surface electromagnetic wave (SEW) generation is of considerable interest in opto-electronic and opto-acoustic applications. Most of the experimental studies of SEW excitation and PSS growth in the irradiated area due to the interference between incident laser wave and SEW were performed for the case of a stationary laser spot. Only few papers are available where authors reported considerable increase in coherence of surface "ripples" when the laser beam is scanned along the surface in order to cover the whole area of a substrate with a periodic relief. See1, for example. Meanwhile, theoretical 1D model developed by Dykhne and Rysev24 predicted velocity resonant effect of PSS formation. Our objective here is to present experimental evidence of the mentioned resonant phenomena and to report 2D effects of SEW/PSS generation in a resonantly moving laser spot and to compare growth efficiency and quality of structures with the results for stationaiy laser spot. Theoretical models of PSS are discussed elsewhere57. Incident laser wave can be scattered by the surface roughness with different spatial harmonics ej in different angles along the surface plane. Whenever roughness spectrum has harmonics with a wave vector q fitting the equation qk+k (k is the tangential component of the incident light wave vector and k is the wave vector of the scattered wave) the instability can occur. Periodic modulation of the energy distribution on the surface due to the interference of the incident and scattered waves can be "recorded" on the surface via vaporization or a similar process. The structure's growth increases scattered wave amplitude and hence the interference field. That, in turn, results in the further development of the relief with spatial period A2ir/q. Note that SEW excitation mechanism, which works for metals and semiconductors, can not be applied to the most of dielectrics. For the case of dielectrics the other origin of the "surface" wave is considered -- the portion of the incident laser wave, refracted by the certain harmonic of the surface roughness within the small angle with respect to the surface plane. It is a convenient way to describe the kinematics of the process with the vector diagram, which actually corresponds to conservation of momentum in the surface plane (Fig.l). Basically, for the p-polarized laser beam the maximum increment for PSS generation in a stationary spot is achieved when SEW wave vector is collinear to k. There are few exceptions though. Firstly, spectrum of surface roughness may contain dominant harmonics (consider scratches, for example). Relatively well developed initial relief forces scattering in the corresponding direction (see diagram), thus leading to the dominant PSS formation. Secondly, there are socalled degenerate structures (qo)8. These PSS are actually formed by two SEWs and due to that have a higher growth increment. This is often a case when angle of laser wave incidence on the surface 8 is large enough (in practice, 0<400), that both SEWs propagate relatively close to k. Finally, the last exception is PSS generation in a moving beam. This case we will discuss further.
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